1. Field of the Invention
The present invention relates to calibration of vector network analyzers in general and in particular to a single port, single connection calibration apparatus.
2. Description of the Related Art
Measurement errors in any vector network analyzer (VNA) contribute to the uncertainty of the device being measured by the VNA. By quantifying these errors, their effects can be drastically reduced. Measurement errors in network analysis can be separated into two categories: random errors and systematic errors. Random errors are non-repeatable measurement variations due to physical change (e.g., noise and temperature changes) and, therefore, are usually unpredictable. Systematic errors are repeatable measurement variations in the test setup itself (e.g., directivity, source match, frequency response, and leakage).
In most measurements made on “devices under test” (DUT) with a VNA, the systematic errors are the most significant source of measurement uncertainty. Therefore, it is desirable to remove these errors from the VNA measurements. This is achieved through a VNA calibration.
The traditional calibration method requires an operator to press a sequence of buttons on a VNA and manually connect and remove at lease three “perfect” calibration components. The VNA measures each component and transfers the accuracy of the standards to the VNA. This calibration process is time-consuming and prone to operator error.
In contrast, an automatic calibration device is useful because it reduces calibration time and reduces the chance of operator error. A prior art automatic calibration device for a one-port VNA is depicted in FIG. 1. The automatic calibration device 116 shown in FIG. 1 requires an operator to connect the calibration device 116 to a test port 104 of the VNA 102 and press a button. The calibration device 116 then automatically calibrates the VNA by connecting the calibration components 120, 122, and 124 to test port 106 through switch 118. The calibration device 116 does not require “perfect” calibration components. Imperfect calibration components 120, 122 and 124 can be used as long as their characteristics (S-parameters) are repeatable and accurately measured. These S-parameters are stored for use by the VNA 102. During calibration, the VNA 102 measures the three calibration components 120, 122 and 124, and these measurement results and the previously stored S-parameter data are used to calculate correction factors. Calibration component 126 is used for verifying the accuracy of the VNA after it has been calibrated.
With the automatic calibration device depicted in FIG. 1, two types of communication are performed between the calibration device 116 and the VNA 102. The first type of communication includes the transmission of digital control signals between the calibration device and the controller 128, while the second type of communication includes the transmission and reception of microwave/radio (RF) signals between the calibration device and the VNA. Thus, communication between the calibration device 116 and the VNA 102 requires at least two ports on both the calibration device and the VNA, along with two separate cables. A first cable 114 carries the digital control signals between a first set of ports (108, 110), while a second cable 112 carries the RF signals through a second set of ports (104, 106). In addition, the calibration device 116 requires an external power supply.
Accordingly, it is an object of the present invention to provide a method and apparatus for calibrating a VNA that requires only one cable and one set of ports for communication between an automatic calibration device and the VNA. It is a further object of the present invention that the calibration device draw its power from the single cable.